1. Field of the Invention
The present invention generally relates to the field of signal processing and, in particular, relates to communication systems that mitigate interference through subspace interference cancellation with little or no knowledge of the characteristics of the interferer or interferers.
2. Description of the Related Art
Wireless receivers most commonly perform interference cancellation using multiple-antenna configurations that facilitate spatial diversity cancellation. Multiple-antenna receivers can “tune” to a small angle of arrival to attenuate interference arriving from other signal sources at different spatial angles.
Strategies for mitigating interference are illustrated in FIG. 1 by showing each received signal from each source as a two-dimensional vector based on two samples per symbol. FIG. 1 shows the signal of interest (SOI) 103 and an interferer 102. In a network that uses a multiple axis scheme to transmit information for multiple users within one symbol, interferers could be other users, other base stations or other noise sources including ones outside of the network.
Projections can be used to mitigate interference, for example by projecting a signal of interest onto a subspace that reduces the power of the SOI but reduces the power of the interferer to a greater extent. This would be the case of projecting the signal of interest 102 and the interferer 103 onto the vector 101. A different projection strategy uses knowledge about the interferer to project both the signal of interest and the interfering signal into a subspace that is orthogonal to the interferer. This subspace is noted by the vector 104 and, because it is at a right angle to the interfering signal vector 102, the resulting interference power should be zero while some signal power should be present in the projection of the signal of interest. Any projection or vector rotation is accomplished via a matrix multiplication.
Conventional communication networks have also used non-spatial-diversity-based interference cancellation methods to improve network capacity. These strategies depend on the receiver knowing the characteristics of the interferer or having system-wide knowledge of waveform properties. For example, a conventional interference cancellation strategy for code division multiple access (CDMA) systems relies on the receiver knowing all codes for the SOI and the interferers.
On the other hand, modern wireless standards as exemplified by the long term evolution (LTE) standard reduce intercell synchronization. For communication networks using the LTE or similar standards, there may not be a way to effectively characterize the significant interferers based on system-wide knowledge of waveform properties. Simultaneous deployment of various standards (GSM, LTE, WiMAX & WCDMA) in a geographical area may require a “blind interference” cancellation strategy, a strategy that can work when no information is known about the interferer, and may need to be applicable to many modulation waveform types. The term “blind interference cancellation” (BIC) typically is used to identify a scheme in which nothing is known about any interferer, while the term “semi-blind interference cancellation” (SBIC) typically is used to identify a scheme in which some information about at least the dominant interferer is known.
Conventional interference cancellation strategies have focused on CDMA wireless networks. Other modern communication networks may utilize orthogonal frequency division multiplexing (OFDM) waveforms. In both CDMA and OFDM networks, users are allocated orthogonal codes or frequencies. Another characteristic of these two networks is that a symbol, containing a burst of bits for the user, consists of a number of samples.
Despite many similarities between CDMA and OFDM protocols, their implementation in a wireless network differs greatly. In the case of CDMA, there is a system-wide timing synchronization (e.g., IS-95) and an inter-cell handling of a user's signal termed a “soft handoff.” Because of these properties, the CDMA receiver will know the codes for nearby base stations, as well as the timing of those codes relative to each other. Conventional CDMA interference mitigation strategies exploit this knowledge to design an SBIC receiver to identify interference associated with known codes from non-SOI sources.
In contrast, an LTE network utilizes OFDM in a design that forgoes receiver knowledge of nearby base stations. Consequently, an interference mitigation strategy cannot rely solely on knowledge of the characteristics of interfering signals. Furthermore, an OFDM receiver typically performs a function not essential in CDMA receivers: channel estimation. That is, a CDMA receiver assigns rake-finger receivers to each of the dominant multi-paths in the channel's impulse response (CIR) without accounting for other aspects of the channel. In the case of OFDM, the receiver typically estimates the relevant CIR span to equalize the signal prior to demodulation and generally makes other use of the channel estimate. Demodulation is the operation in OFDM receivers that extracts the information bits intended for the user.
Four common scenarios in wireless communication are prominent enough to consider in evaluating strategies for interference cancellation. These four scenarios are: 1) uncooperative interferers; 2) similarly modulated interferers with unknown waveform properties; 3) similarly modulated interferers with known waveform properties; and 4) unknown spatial conditions which may result in single antenna conditions (e.g., “the death grip”).
Scenario #1 refers to adjacent channel interferers. In wireless communication deployments with tight spectrum availability, there may be significant energy spilled from adjacent communication links that limit achievable performance. In such a case, the adjacent transmitter is said to be “uncooperative” since the receiver has no knowledge of the transmitter's transmission waveform properties. This geographically adjacent channel's transmitter can be assumed to be of a different waveform design, but uncooperative as to its waveform parameters.
The LTE standard increases network capacity through “intentional interference” resulting in interference scenario #2. In this scenario, termed HetNet in LTE, the interferer is modulated similarly to the desired or intended transmission, but conditions may be such that only this similarity is known. Information is not available to the receiver as to what specific parameters were used by the interfering station to create its waveform, if the symbols are synchronized to the same clock as the intended transmission, or if the symbols are of the same duration as the intended transmission.
Scenario #3 involves a network that achieves higher throughput to users through synchronized interference. In this scenario, the receiver knows some information about the interferer that can include certain of the interfering signal's waveform properties.
Scenario #4 relates to a loss of designed multiple antenna diversity. This scenario received popular attention when a so-called “death grip” on a smartphone reduced receiving performance. The multiple antenna configuration was such that a left-handed grip of the phone could lead to an effective single-antenna reception. More generally, “shadowing” by the hand or body can cause a similar loss of spatial diversity. In this scenario #4, a desirable interference cancellation scheme would work to sustain rates similar to those achievable with one antenna and no interferer.